Security in Wired and Wireless Networks
In wired communication networks, signals are mostly confined in physical transmission media, such as conductive wires and optical fibers. Hence, an eavesdropper can only access the signals by physically attaching to the media.
In contrast as shown in FIG. 1, signals 105 in a wireless network 100 are broadcast by a transmitter node (Alice) 101 over wireless channels. The signals are accessible to any receiver node, e.g., a legitimate user Bob 102 and an eavesdropper Eve 103, within a certain radio propagation range 110. Therefore, security is an extremely important issue in wireless communications.
Public-Key Cryptography
To achieve secure wireless communication, public key cryptography is widely used. The asymmetric structure of the keys does not require a perfectly secure wireless channel to exchange a pair of keys between the transmitter and the receivers. The pair of keys includes a private key and a public key. To operate the network, a public key infrastructure (PKI) generates, distributes and maintains the public keys, in which a trusted party, i.e., a certificate authority (CA), binds all the public keys with a receiver identity and issues a public key certificate to the receiver.
To establish secure wireless communication, the transmitter first verifies the public key certificate of the receiver. After the public key is verified, data are then encrypted using the receiver's public key. The data can only be decrypted using the corresponding private key.
However, for many wireless networks, access to the PKI is difficult, or completely unavailable. In such cases, secure communication in wireless networks becomes challenging. Given this, guaranteeing security in a wireless network is of great interest to users of wireless networks.
Channel Reciprocity
FIG. 2 shows the concept of conventional wireless channel reciprocity 200. The principle of channel reciprocity states that channels between two wireless nodes are reciprocal, i.e., the channel 201 from Alice 101 to Bob 102 has substantially the same characteristics as the channel from Bob to Alice 202, i.e., Hab=Hba, for a predetermined frequency range at a predetermined time instance.
The reciprocity of wireless channels enables two nodes to generate private keys based on the channel responses of the reciprocal channels. However, due to noise, interference and hardware impairments, the channels are not perfectly correlated. Therefore, private keys generated independently by the nodes do not always match. If a conventional cryptography technique is used for data to be transmitted, the two keys must be identical, or otherwise the receiver cannot decrypt the data correctly.
Therefore, it is desired that each key has a low bit mismatch rate (BMR). The BMR is defined as a ratio between the number of mismatched bits to the total number of bits in each key.